GAMG vs. PCG in fvSolution

grandgo · July 21, 2011, 05:39

hi foamers,

i'm working with two domains of exactly same geometry and case configurations. one domain has 150000 cells, the other about 6*10^6 cells.

in the coarse domain case the GAMG solver needs about 30 iterations for one timestep (nCellsInCoarsestLevel = 1000). on the other hand, if i choose PCG the calculation needs 230 iterations.

in the fine domain case it's the opposite: PCG needs about 1000 iterations but GAMG does need more (maxIter limitation at 1500, various nCellsInCoarsestLevel).

what's your experience with GAMG? any tips what i could try?

best regards
grandgo

Antony_P · November 13, 2015, 09:58

Hi,

Did you find any answer?

I am solving a external flow simulation. When i increase the number of layers and decrease the min thickness close to 9.6e-06,in a GAMG solver the iterations reach close to 1000 and crashes but on the other hand PCG works very well.

Is there any reason behind.

Thanks,
Antony

KateEisenhower · March 7, 2017, 08:38

Hi,

Thanks for opening this topic. I have two suggestions:

1) Some people here on the forum say nCellsInCoarsestLevel shouldn't be above 50. On my cases I always make sure to make a "nCellsInCoarsestLevel-study".

2) I think your post would be even more meaningful if you would tell us the calculation time/iteration or per timestep. The number of iterations alone is not so meaningful from my point of view.

Best regards,

Kate

July 21, 2011, 05:39	GAMG vs. PCG in fvSolution	#1
grandgo Member Join Date: Oct 2010 Location: Stuttgart Posts: 35 Rep Power: 16	hi foamers, i'm working with two domains of exactly same geometry and case configurations. one domain has 150000 cells, the other about 6*10^6 cells. in the coarse domain case the GAMG solver needs about 30 iterations for one timestep (nCellsInCoarsestLevel = 1000). on the other hand, if i choose PCG the calculation needs 230 iterations. in the fine domain case it's the opposite: PCG needs about 1000 iterations but GAMG does need more (maxIter limitation at 1500, various nCellsInCoarsestLevel). what's your experience with GAMG? any tips what i could try? best regards grandgo

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November 13, 2015, 09:58		#2
Antony_P New Member Antony Premkumar Join Date: Jul 2015 Posts: 19 Rep Power: 11	Hi, Did you find any answer? I am solving a external flow simulation. When i increase the number of layers and decrease the min thickness close to 9.6e-06,in a GAMG solver the iterations reach close to 1000 and crashes but on the other hand PCG works very well. Is there any reason behind. Thanks, Antony

March 7, 2017, 08:38		#3
KateEisenhower Senior Member Join Date: Mar 2015 Posts: 250 Rep Power: 12	Hi, Thanks for opening this topic. I have two suggestions: 1) Some people here on the forum say nCellsInCoarsestLevel shouldn't be above 50. On my cases I always make sure to make a "nCellsInCoarsestLevel-study". 2) I think your post would be even more meaningful if you would tell us the calculation time/iteration or per timestep. The number of iterations alone is not so meaningful from my point of view. Best regards, Kate